1. Field of the Invention
The present invention relates to a PTC (positive temperature coefficient) thermistor adapted to be employed in a heating circuit, an overcurrent protection circuit or the like and a method of producing such PTC thermistors, and a resistor adapted to be employed in a magnetic erasure circuit for a television and a radiator for a fan forced heater, a water-proof heater or the like.
2. Description of Related Art
FIGS. 20 and 21 show existing PTC thermistors. Referring to FIG. 20, electrodes 81 and 82 made of a silver compound which performs ohmic contact, such as a compound of silver with zinc, a compound of silver with tin, etc., are formed on upper and lower surfaces of a substrate 80. Referring to FIG. 21, double-layered electrodes 91 and 92 are formed on upper and lower surfaces of a substrate 90. The electrode 91 is composed of an underelectrode 91a formed on the surface of the substrate 90 and a silver electrode 91b formed thereon. The electrode 92 is composed of an underelectrode 92a and a silver electrode 92b likewise. The reasons why a silver compound is used as the electrodes 81, 82, 91b and 92b are that a lead terminal is soldered with silver easily and that silver is excellent in ohmic contact.
Using a silver compound as an electrode, however, has a disadvantage that the main constituent of the electrode (silver) is likely to react with chlorine ions of water in air and be ionized, causing silver ions to migrate. More specifically, the substrates 80 and 90 are ceramic, and the surface potential of each substrate 80 or 90 is not uniform, whereby ionized silver of the electrodes 81 and 82, or 91 and 92 migrates on the surface from a higher potential area to a lower potential area. This migration of silver ions may cause a short circuit.
Also, since the substrates 80 and 90, and the electrodes 81, 82, 91 and 92 are exposed, these thermistors are likely to be affected by external force and become defective easily. For example, the substrates 80 and 90 may develop cracks and chips during the production or the transport. Also, the electrode 81, 82, 91 or 92 may peel off the substrate 80 or 90, causing a spark at the time of impressing a voltage. Thus, from an electrical point of view, such thermistors are not reliable.
Further, since the sides of the substrates 80 and 90, which are porous, are exposed, harmful gases such as a chlorine gas, a halogen gas, etc. are likely to penetrate into the substrates 80 and 90 through the sides, deteriorating the substrates 80 and 90. Such a deteriorated substrate impairs the performance of the PTC thermistor.
From thermistors as shown in FIGS. 20 and 21, generated heat leaks, and when such a thermistor is employed in a magnetic erasure circuit for a television, a picture received by the television shakes. When such a thermistor is employed in a radiator for a fan forced heater, a water-proof heater or the like, the thermistor interferes with an improvement of the radiator in temperature-rise characteristic. Further, a PTC thermistor is conventionally housed in an insulating case having holes through which terminals are protruded, and there is fear that substances which deteriorate the PTC thermistor, such as flux, chlorine, etc., may penetrate into the case through the holes. Furthermore, the PTC thermistor may be displaced from a set position and may contact with the inner wall of the insulating case. In this case, a gas is generated from the insulating resin of the case with the generation of heat by the PTC thermistor, and this gas may deteriorate the PTC thermistor.